Cannot log in after upgrade to 2.8.4

I upgraded to 2.8.4 about 2 weeks ago. Shortly afterwards I could no longer log into the admin screen /wp-admin. I get the message “You do not have sufficient permissions to access this page”.

There a loads of posts with the same message, but none of their resolutions have worked for me. eg. resetting the password directly in the database.

I have now restored the files and database back to 2.7.1 from the backups I took. But I still get the same error which I just can’t work out why!

So I’m a bit stuck now as I can’t change anything on the website.

Any help appreciated.

Thanks.